The invention relates to methods for fabricating substrates, in particular for optics, electronics, or optoelectronics, and also to substrates obtained by such methods. More particularly, the substrates are also suitable for use in fabricating light-emitting and laser diodes.
In the technology field of optics, electronics, and optoelectronics, it is often desirable to obtain substrates that comprise a working layer. Two types of methods are currently known for making such substrates. One method includes transferring a working layer from a source substrate onto a support substrate. The other method includes depositing a working layer onto a support substrate by deposition techniques such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOVCD), and the like.
Source substrate materials from which a working layer can be transferred, however, are very difficult to obtain or are otherwise not available. This applies in particular to monocrystalline gallium nitride, which is not available in the form of a solid single crystal of quality, nor is it available with a diameter that is satisfactory, and not available at a reasonable price. Consequently, monocrystalline gallium nitride is typically grown only by a heteroepitaxial technique.
Furthermore, growth of a working layer on a substrate by deposition techniques is not yet satisfactory. There are known techniques for growing a working layer on a seed layer itself carried by a support, in which there is often a need to remove the support substrate in order to obtain the final product. Although techniques for removing the support are known, there are drawbacks. For example, FR 2,787,919A, which is incorporated herein by reference thereto, describes eliminating such a substrate by a mechano-chemical thinning technique. However, all techniques of removing the support by etching or some equivalent technique are undesirable since they lead to significant losses of material, which often times is expensive. U.S. Pat. No. 6,114,188, incorporated herein by reference, also describes a technique for separating a complex transition metal oxide (CTMO) film made by deposition, in which a special treatment is performed on the native substrate from which the film is to be grown, and then the deposited film is detached from the native substrate. Nevertheless, that technique is also undesirable since it runs the risk of compromising proper initiation of film growth and thus leads to either a loss of yield or to a deposited layer of poor quality.
Additionally, it is known that sapphire, silicon carbide and {111} silicon can be used as seed substrates for the deposition of gallium nitride by heteroepitaxy. However, sapphire is an electrical insulator, which is a disadvantage in certain applications, and monocrystalline silicon carbide presents the drawbacks of being expensive and difficult to obtain in large diameters. Furthermore, as sapphire is an electrical insulator, if it is retained in the form of a solid support, then it becomes necessary for any electrodes needed for the intended application of the working layer to be provided solely on the working layer itself, which can give rise to problems of available space (for example, if two electrical contacts are to be made on a front face, i.e. on the free surface of the working layer).
In prior art techniques, monocrystalline silicon carbide or sapphire were used to serve both as a support and as a growth seed for the working layer. One drawback from such prior art techniques exists when the working layer is used to form light-emitting diodes (LEDs), using a solid silicon carbide or sapphire support means, namely, that it is not possible to satisfactorily control the positions of electrical contacts, the extraction of the light emitted by the diode, or the use of a reflecting surface, etc
Although {111} silicon may be ideal for use as a substrate due to its very widespread use, the fact that it is inexpensive and available in large diameters, problems have arisen during attempts to deposit gallium nitride on {111} silicon using the standard technique of MOCVD at about 1000xc2x0 C. to 1100xc2x0 C. Such problems include dislocations forming in he thin layer of gallium nitride at a concentration in excess of 108 per square centimeter (cm2). Moreover, if {111} silicon is used as a support, i.e. in thick form, then cracking is observed in the working layer because of poor matching in terms of thermal expansion.
In other prior art techniques, attempts have been made to deposit gallium nitride directly on solid gallium nitride or indeed on neodymium gallate or on indium gallate. However, solid gallium nitride is expensive and those techniques are not mature.
Thus, improvements in such methods for fabricating a substrate are desired. Also needed is a method that overcomes the difficulties associated with substrate materials. Certain new and useful improved methods are provided by the present invention.
The invention provides new ways for fabricating substrates for use in the fields of optics, electronics, or optoelectronics. In particular, in accordance with the invention a method is provided for fabricating a substrate generally comprising transferring a seed layer onto a support, depositing a working layer on the seed layer to form a composite substrate, and detaching the seed layer and the working layer from the composite substrate to form a substrate. Advantageously, the support substrate comprises material having a thermal expansion value of about 0.7 to 3 times the coefficient value of the working layer. Preferably, the seed layer comprises material suitable for accommodating the thermal expansion of the support layer and the working layer.
Also in accordance with the invention, a seed layer is transferred onto a support by molecular adhesion in an adhesion interface, a working layer is deposited onto the seed layer to form a composite substrate, and the seed layer and the working layer are detached from the composite substrate by the application of stress. Advantageously, detachment of the support from the seed layer and working layer may be accomplished by the application of stress which therefore provides for the possibility of recycling and reusing the support substrate.
The method may also include a source substrate comprising a seed layer and a weakened zone. The seed layer may be detached from the source substrate at the weakened zone, and transferred onto the support substrate.